Steven Hoskin had strong feelings about his killers. They had abused, exploited and humiliated him over a year, taking his money, treating him as their slave and making him wear his own dog's collar and lead. Eventually, having forced him to swallow 70 painkillers, they took him to the top of a railway viaduct and made him hang from the railings as one member of the gang, a girl aged 16, stamped on his hands until he fell 30 metres to his death.

Yet these were the people the 38-year-old, who had severe learning disabilities, had boasted excitedly of counting as friends. "He thought they were the cat's whiskers," says Morley Richards, who had known Hoskin before he met the group. "He would say, 'They're my mates, I've got my own mates now.'"

Hoskin's case is extreme, but the phenomenon of learning disabled people being groomed by those who pretend to be their friends before being exploited by them financially, physically or sexually – "mate crime", as it is sometimes known – is far from rare, experts say, and appears to be on the increase. As more individuals are given the chance to live independently, the unwelcome side effect is that they are more likely to fall prey to criminals. Hoskin was a case in point: he had left the tiny Cornish village of Maudlin, near Bodmin, where he had grown up, and was thrilled to have his own bedsit in the market town of St Austell, where he made his new "friends".

The Association for Real Change (ARC) has been researching mate crime for the past year in Calderdale, west Yorkshire, and in north Devon, after a groundswell of concern among its members who are service providers for people with learning disabilities. Examples it has been told about range from perpetrators routinely going to a victim's house and clearing their cupboards of food and alcohol before leaving them to clear up the mess, to instances of people being persuaded to part with their benefits.

Women can be sexually exploited by men who claim to be their boyfriend, says David Grundy, who runs the Calderdale project. "They might be told, 'We're a couple and we haven't got any money – if you sleep with my mate he'll pay us.'" In other cases, someone with learning disabilities may be asked to look after a package that contains drugs and end up being beaten up as a result, or go shoplifting with their new-found friends carrying a weapon, only to get caught by police.

The victim may not realise that what is happening is wrong. "There can be a feeling of, 'He's my friend, that's what friends do,'" says Grundy. "People with learning disabilities have fewer friends. For some, any friends is better than no friends, even if they're spending all your money.

"It involves a lot of issues [around] self-belief and self-worth: thinking it's all right for people to walk all over them all the time, because that's what's happened to them the whole of their lives."

Rod Landman, from the north Devon project, likens the situation to domestic violence. "The primacy of the relationship can be more important than what's happening inside it. People are prepared to put up with all sorts of crap to keep a relationship that may be the only one they have apart from with someone who's being paid to be with them." As a result, victims shy away from reporting such hate crime incidents to the police, or indeed anyone. Every service provider that Landman talks to will tell him of cases, he says, but no one with learning disabilities will do the same.

Some families and frontline social care staff are still unaware of what constitutes a disability hate crime and what to do when one happens, says Grundy. Abusive relationships may get flagged up to adult safeguarding teams, but their primary aim is to keep the individual safe by removing them from the situation, rather than report those committing the crimes. This means that perpetrators remain free to target others.

As cuts lead to the closure of day centres and potentially less support for vulnerable people, there are fears that the situation could get worse. Gavin Harding, vice-chair of the National Forum for People with Learning Disabilities, remembers the shock and anger he felt five years ago when he realised that someone he believed to be a friend had taken his cheque book and, together with another man, forged a cheque for £500.

"It was the fact they took advantage of me," he says. "It felt awful. You feel you can't trust people after that."